This invention relates to security systems, in particular those that monitor whether an opening is opened or secured. A switch is maintained at each opening in the closed position when the opening is secured and opens when the door or window is opened. Wireless systems, such as that described in U.S. Pat. No. 4,367,458 (Hackett), assigned to the same assignee as the present invention, must overcome problems caused by interference from other communication systems and multipath. Ultrasonic systems, in particular, are confronted with acoustical interference and are restricted in speed by the relatively slow speed of sound. A wired alarm system avoids these problems. Furthermore, many facilities are already wired, facilitating installation of a wired alarm system, such as the present invention.
It is a common practice in wired alarm systems to connect the switch contacts at the openings in series so that if one or more of the contacts opens, the circuit is broken. The interruption in the current, causes an alarm. Also, an alarm would result if the interconnecting wire is cut.
Prior to arming such an alarm system, all the openings must be secured. If an attempt is made to arm the system while a window or door is open, an immediate alarm would result. Usually the system will have an indicator to inform the user that one or more of the openings has not been secured. If that is the case, the alarm user would have to search the premises to find which of the openings are not secured. In a large system this can be a time consuming task.
A more serious problem encountered by users of alarm systems occurs when there is a false alarm due to an intermittent contact. It is almost impossible to find the faulty contact until its performance deteriorates to the point where it has become continuous. By then the aggravation has often reached the point where the user has asked the alarm company to remove the system.
To minimize this problem, systems exist which have been arranged in a zoned system, the contacts are grouped together, each group returning through a separate input to the control panel via its own separate wire.
If an opening has not been secured or there is an intermittent contact, the search can be confined to the particular zone. The ideal system would have one zone per contact. There are times when this could be done, but in general it would be very expensive since it would require a wire to be run from each protected point back to an input of the control. Besides requiring a large amount of wire, one input to the control would be required for each monitored opening.